Method for the manufacture of porcelain

ABSTRACT

The invention relates to a porcelain with a transparency and strength properties of a hard porcelain, characterized by a phosphate-containing glass phase as well as the predominance of unreacted quartz bodies [sic; grains] in the matrix. The invention relates to a porcelain batch composition as well as to a method for the manufacture of the porcelain.

This is a division, of application Ser. No. 08/157,634, filed on Nov.24, 1993, now U.S. Pat. No. 5,455,210.

The invention relates to a porcelain, particularly a tablewareporcelain, a porcelain batch composition and a method for themanufacture of the porcelain.

Currently three tableware porcelain types are distinguished in Germany:hard porcelain, vitreous porcelain and bone porcelain.

Hard porcelain as a rule is made from a batch composition consisting of50% clay material, 25% feldspar and 25% quartz, which is fired at1350°-1450° C., where the sharp fire takes place in a reducingatmosphere. In principle the clay material used is a light-burningfiring kaolin. Feldspar acts as a flux and it is usually used in theform of a potassium feldspar or as a potassium-sodium feldspar.

The fired body of hard porcelain, which consists essentially ofsecondary mullite, quartz and glass phase, exhibits a high transparencyand a high degree of whiteness. Furthermore, it is characterized by ahigh mechanical strength, good thermal and electrical properties andsufficient resistance to corrosion.

A decrease in the quartz portion results in an improvement in resistanceto temperature changes; an increase in the quartz portion can lead to anincrease in mechanical strength. If the kaolin portion is increased tothe detriment of the quartz, the transparency decreases.

For the preparation of the composition for the manufacture of hardporcelain, the raw materials are finely divided and mixed with water.The shaping step is followed by drying and glazing. The glazing isapplied to the raw body or to a body which has been spoiled by too highannealing temperatures at 900°-1100° C. in an oxidizing atmosphere. Thehard porcelain is fired primarily with oxidation at 20°-900° C., withoxidation at 900°-1000° C., and with reduction at 1000°-1450° C. bysharp firing. During the cooling the atmosphere is neutral or oxidizing.During the firing the composition exhibits a very good preservation ofshape. Deformations during firing are minimal.

The following property values are required for hard porcelain:

    ______________________________________                                        Water uptake           0                                                      Raw density            2.3-2.4 g/cm.sup.3                                     Pressure resistance irregular                                                                        450-550 MPa                                            Impact bending resistance irregular                                                                  23 MPa · m                                    Bending resistance irregular                                                                         40-70 MPa                                              Average linear heat expansion                                                                        4.5 × 10.sup.-6 K.sup.-1                         coefficient (20-500° C.)                                               Degree of whiteness irregular                                                                        >70%                                                   Transparency with d = 1 mm                                                                           2-10%                                                  Resistance to temperature change                                                                     >165 K.                                                Primary components:                                                           glass phase            64-65%                                                 mullite                27 ± 1%                                             quartz                 7 ± 0.5%                                            cristobalite           2 ± 0.5%                                            ______________________________________                                    

Vitreous porcelain differs from hard porcelain in its higher feldsparand quartz portions as well as, possibly, in that it has a certaincontent of fatty, white-burning clays in the batch composition,resulting in lower firing temperatures. Accordingly, the values of theproperties of vitreous porcelain do not correspond to the high standardof hard porcelain. Vitreous porcelains are fired primarily withoxidation and as a result they often are the color of ivory. Oneadvantage resulting from the lower firing temperatures is that, incomparison to hard porcelain, a broader range of colors for decorationsor glazings is available.

Bone porcelains (bone china) are characterized by a high transparency, acreamy white color, a high degree of whiteness and good mechanicalstrength. The highly transparent body of bone porcelain is howeverrelatively brittle. As a rule the batch composition consists of 20-45%bone ash, 20-45% cornish stone and 20-35% kaolin.

Bone ash is prepared from defatted leached bones by calcining withaddition of air. It contains as essential mineral phase calciumphosphate. Cornish stone is a partially kolinized potassium and sodiumfeldspar-containing stone which contains besides feldspar quartz,kaolin, mica and a small quantity of fluorspar.

The composition for the manufacture of bone porcelain is difficult toprocess particularly because of the low kaolin portion and it is moredifficult to fire than other porcelain compositions. The firing occursin an oxidizing atmosphere at 1200°-1300° C., and the first firing stepis the glazing. The glaze is applied to the densely fired body. Thefiring for glazing is conducted at 1000°-1140° C. The relatively lowfiring temperature results in glazings which are softer and less scratchresistant than the conventional porcelain glazings. Because of the batchcomposition deformations occur relatively frequently during firing.

Most of the high manufacturing costs of hard porcelain are the result ofthe sharp fire temperatures. As a result, attempts have been made tochange the batch composition so that lower fire sharp fire temperaturescan be used while still guaranteeing that the values of the propertiesof hard porcelain are obtained. For example attempts have been made toimprove the sintering properties using mineralizers which have an effecton quartz dissolution, such as ZnO and chalk (Silikattechnik 39 (1988),No. 7, pages 228-231). The known batch composition has, however, beendesigned especially for raw materials which occur only regionally, suchas particular feldspar sands, where the sharp fire temperature is stillrelatively high at 1350° or 1380° C.

In other countries, porcelains are classified differently, but here toothe batch compositions have been defined with narrow ranges and lead tocertain properties of the products. A minor omission or change in theselected batch compositions and/or a change in the particular firingconditions as a rule results in unusable products. For this reason it isa traditional principle to change nothing or make only insignificantlysmall changes to the operating conditions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a photograph of a disclosed embodiment of porcelain made inaccordance with the present invention;

FIG. 2 is a photograph of vitreous porcelain.

FIG. 3 is a photograph of hard porcelain.

FIG. 4 is a photograph of bone china.

The problem of the invention is to formulate a manufacturing process toproduce a porcelain which has the properties of a hard porcelain but ismore economical and tolerates greater variations of the firingconditions while maintaining the relatively simple manufacturingtechnology and the usual firing equipment.

The problem is solved by the use of a batch composition with thefollowing chemical composition:

18-27 wt. % Al₂ O₃

50-65 wt. % SiO₂

0.1-0.3 wt. % Fe₂ O₃

0.02-0.1 wt. % TiO₂

5-8 wt. % CaO

0.15-0.3 wt. % MgO

0.5-2.0 wt. % Na₂ O

2.6-4.0 wt. % K₂ O

3.5-6.0 wt. % P₂ O₅

Raw materials of high purity with a Fe₂ O₃ and a TiO₂ content of lessthan 0.4 wt. % are used for the batch composition which is made up ofclay material, feldspar, quartz and bone ash.

The particle size distribution of the batch compositions is selectedadvantageously as follows:

    ______________________________________                                        <40        μm        95-100 wt %                                           <20        μm        82-97 wt %                                            <10        μm        67-80 wt %                                            <5         μm        50-65 wt %                                            <2.5       μm        36-55 wt %                                            <1         μm        24-35 wt %                                            <0.5       μm        15-25 wt %                                            ______________________________________                                    

The particle size distribution of quartz should be as follows:

    ______________________________________                                        <40.00      μm       88-100 wt %                                           <20.0       μm       55-86 wt %                                            <10.0       μm       30-55 wt %                                            <5.0        μm       18-30 wt %                                            <2.5        μm        8-15 wt %                                            <1.0        μm        3-9 wt %                                             <0.5        μm        1-6 wt %                                             ______________________________________                                    

Water is admixed with the batch composition so that a processablecomposition is formed. Objects are formed from the compositions, andthen dried and fired. The firing occurs at a temperature between1120°-1220° C. The fired product is a low-[temperature]-firedtransparent porcelain with the mechanical properties of hard porcelainand a firing color which depends on the oven atmosphere which [color]has been regulated. In an oxidizing atmosphere one obtains a creamywhite [color] corresponding to bone china and in a reducing atmospherethe gray-speckled white hard porcelain. By means of appropriatelyregulated oven atmospheres it is possible to obtain intermediate whitetints. However, the remaining properties of the low-[temperature]-firedporcelain in all cases correspond to those of hard porcelain.

The batch composition according to the invention permits the followingadvantages:

1. Low firing temperature:

This results in an economic advantage which permits a considerableenergy saving and which multiplies the life span of firing adjuvants.

2. The batch composition is suited for an oxidizing or a reducing firingatmosphere.

3. The composition is suited for rapid firing (4-6 h) and normal firing(10-18 h).

4. The product exhibits a high transparency.

5. The composition presents only small deformation during firing. Thisresults in great advantages for design and production; a broad firingspectrum can be used, and use with oxidizing or reducing atmosphere ispossible. A body with low deformation during firing makes it possible touse, as with hard porcelain, the considerably less complicated glazingapplication onto the absorbing body.

6. The values of the physical and chemical properties of transparency,bending-rupture resistance, impact resistance, hardness, acidresistance, washing-machine resistance, suitability for microwave ovens,freedom from heavy metal [content] correspond to the properties of hardporcelain fired at 1400° C.

With the new batch composition an alternate product for either hardporcelain or bone china is created. The advantage of this product isthat depending on the atmosphere the white color can be influenced andthus it is possible to produce appearances which have never beenobtained before.

The targeted decrease in the firing temperature to 1200° C., forexample, could be reached by a rational change of the composition.

The purpose here was to be able to use the manufacturing technologieswhich are conventionally used for hard porcelain, and not to have to usethe expensive and complicated manufacturing method of bone china. Animportant prerequisite here is a good stability under load of thecomposition during firing. The new composition has a carefullydetermined portion of flux, quartz and plastic raw materials, to preventa strong deformation during firing.

The matrix structure of the body of the low-[temperature] firedporcelain according to the invention is clearly distinguishable fromthat of hard porcelain and that of bone china. Besides the completelyunreacted quartz, primary mullite is present which is formed fromfeldspar and kaolin, and there is also tricalcium phosphate andanorthite. A glass phase surrounds all of this, and it is responsiblefor the high transparency of the body and contains an enriched contentof P₂ O₅. As a rule the phases of hard porcelain consist of primary andsecondary mullite, glass phase, and residual quartz in various ratios.The residual quartz portion, and whether there is any residual quartz atall, depends on the firing temperature and the time in the sharp fire.The conversion of primary mullite to secondary mullite is alsoinfluenced by the firing curve. The phases of bone china consist ofanorthite, calcium orthophosphate and quartz, with only a few quartzparticles which have not reacted completely because of their size.

The matrix of the porcelain according to the invention (FIG. 1) consistsof quartz, anorthite, mullite, glass and pores.

The matrix of vitreous porcelain (FIG. 2) consists of quartz, mullite,glass and pores.

The matrix of hard porcelain (FIG. 3) consists of residual quartz,mullite, glass and pores.

The matrix of bone china (FIG. 4) consists of anorthite, calciumorthophosphate and quartz, with only a few quartz particles which havenot completely reacted because of their size.

It is known that as the granular fineness of the quartz and of thefeldspar increases, there is an increase in vitrification and thus intransparency so that it is possible to lower the firing temperature.Furthermore it is known that the melting process can be influenced, forexample, to achieve approximately the same melt phase portion in thebody with rapid firing compositions as with normal firing compositions.

In comparison the invention has taken another route and surprisingly ithas attained a transparency and strength levels corresponding to thoseof normal hard porcelain with a relatively low quartz dissolution and arelatively low melt portion .

The invention thus contains a selected batch composition which, atrelatively low temperatures, can be fired either by rapid firing or bynormal firing, with either oxidation or reduction. It is essential thatas a function of the oven atmosphere the color of the body can beinfluenced, specifically with colors ranging from a creamy white in anoxidizing atmosphere to a blue-gray speckled white in a reducingatmosphere, and all intermediate colors can be produced depending on theoven atmosphere used during the firing.

The relatively low firing temperature and the possibility of selectingthe atmosphere also provide the possibility to extend the glazingpalette of usable glazings considerably. The composition can be coloredby the addition of dissolved or pigment-like color additives and in thatcase the result is a colored transparent body. Furthermore, the basiccomposition can be covered to produce a white or colored cover with allthe glazing variants conventionally used in ceramics, for example,transparent, opaque, covering, crystal separating, matt glazings.

It is surprising that in spite of the batch composition which isdifferent compared to conventional porcelain and in spite of theresulting different matrix structure of the porcelain according to theinvention a very broad firing range is available and the firing can beconducted simply and with known firing equipment, where the stabilityunder load of the shaped body during the entire temperature treatment isoptimal.

I claim:
 1. A method for the manufacture of porcelain having a porcelainmatrix consisting essentially of mullite, glass phase, quartz grains andpores comprising the steps of:preparing a porcelain batch compositionfrom raw materials, said batch composition consisting essentially of:18-29 wt. % Al₂ O₃ 50-65 wt. % SiO₂ 0.1-0.3 wt. % Fe₂ O₃ 0.02-0.1 wt. %TiO₂ 5-8 wt. % CaO 0.15-0.3 wt. % MgO 0.5-2.0 wt. % Na₂ O 2.6-4.0 wt. %K₂ O 3.5-6.0 wt. % P₂ O₅ ; andfiring the batch composition.
 2. Methodaccording to claim 1, wherein the sum of Fe₂ O₃ and TiO₂ in the batchcomposition is less than 0.4 wt. %.
 3. Method according to claim 1,wherein the batch composition comprises clay material, feldspar, quartzand bone ash.
 4. Method according to claim 1, wherein the particle sizedistribution of the raw materials of the batch composition is asfollows:

    ______________________________________                                        <40        μm        95-100 wt %                                           <20        μm        82-97 wt %                                            <10        μm        67-80 wt %                                            <5         μm        50-65 wt %                                            <2.5       μm        36-55 wt %                                            <1         μm        24-35 wt %                                            <0.5       μm        15-25 wt %.                                           ______________________________________                                    


5. Method according to claim 3, wherein the quartz in the batchcomposition has the following particle size distribution:

    ______________________________________                                        <40.00      μm       88-100 wt %                                           <20.0       μm       55-86 wt %                                            <10.0       μm       30-55 wt %                                            <5.0        μm       18-30 wt %                                            <2.5        μm        8-15 wt %                                            <1.0        μm        3-9 wt %                                             <0.5        μm        1-6 wt %.                                            ______________________________________                                    


6. Method according to claim 1, wherein the firing is conducted in areducing atmosphere thereby producing a gray-specked white porcelain. 7.Method according to claim 1, wherein the firing is conducted in anoxidizing atmosphere thereby producing a cream colored porcelain. 8.Method according to claim 1, wherein the firing takes from 10-18 hoursin a conventional normal firing process.
 9. Method according to claim 1,wherein the firing takes from 4-6 hours in a conventional rapid firingprocess.